WATER-TABLE CONTROL ON RILL INITIATION AND IMPLICATIONS FOR EROSIONALRESPONSE

On sloping sandy agricultural soil sites near Toronto, Canada, summer storms adequate to generate runoff occur frequently, but rill developm ent occurs mainly in spring when snowmelt or rainfall-induced runoff o ccurs above frozen subsoil. This suggests that on low and moderate slo pes on these soils rill initiation is controlled primarily by hydrauli c impedance close to the surface, rather than critical hydraulic condi tions in runoff. Laboratory flume experiments were carried out on 10 m slopes at 1.5 degrees, 5 degrees and 9 degrees with loamy sand/clay c omposite soil sample to test this hypothesis. Runoff with hydraulic co nditions adequate for rill initiation occurred rapidly in most tests, but on 1.5 degrees and 5 degrees slopes little knickpoint scour or sed iment transport occurred before water table development. This coincide d with reduced surface soil strength, knickpoint scour and marked incr ease in sediment discharge, particularly on 5 degrees slopes where inc rease was 20- to 30-fold. Further increase in sediment discharge occur red when water tables reached the surface. On 9 degrees slopes runoff occurred more quickly, with higher hydraulic values. Significant rill incision and sediment discharge occurred well before water table devel opment, and ultimately reached much higher values than on lower slopes . Results show that soil erodibility can change dramatically over shor t time periods during storms due to soil moisture conditions, and that the presence of a hydraulic impedance close to the surface which caus es a perched water table to develop can strongly influence rill incisi on and sediment transport. The influence is unlikely to be marked on s oils which are very erodible regardless of moisture conditions, or on extremely resistant soils. It will also be Limited on very gentle or s teep sites, but can be a significant factor in rill development on int ermediate slopes. (C) 1998 Elsevier Science B.V. All rights reserved.